1. Field of the Invention
The present invention relates to apparatus for use in digital networks of a type which connect a plurality of computers to form a computer network and more particularly to apparatus for transferring data between subscriber loops and a time-domain multiplexed communication link.
2. Description of Related Art
Many businesses have computer networks which extend over large geographic areas. The most economical manner for implementing such networks utilizes existing telephone lines to connect the various computers in the network. Computer networks that utilize the telephone system are known to the prior art. However, these computer networks are less than ideal. Problems arise from the use of transmission facilities which were designed for voice quality transmissions between two points and from the inclusion of components which are owned and maintained by separate commercial entities. However, because of the cost savings which are inherent in the use of the existing telephone system, it is desirable to adapt such network equipment for acceptable digital data transmission, if at all possible.
Existing computer networks suffer from a number of drawbacks. For example, prior art computer networks which utilize the telephone system are difficult to reconfigure either with regard to the speed at which data is transmitted within the telephone system or with regard to the topological configuration of the computer network. In prior art computer networks, telephone company technicians must be dispatched to various locations within the telephone system to effect a data rate change in the computer network. If the new rate is not one of a small number of rates for which the equipment is designed, the actual interface circuitry within the telephone system must often be changed. As a result, it is practically impossible to change data rates in response to short term changes in the quality of the telephone lines carrying the data.
Short-term increases in the noise on specific telephone lines are common. The telephone lines used for carrying the user's computer data are, in general, identical to the lines used to carry voice communications within the telephone system. These lines are bundled into cables with similar lines. The noise level on these lines depends upon a number of factors including the nature of the transmissions on lines bundled with those used for the computer network along routes which include a variety of telephone company switching equipment.
To change the network speed or configuration, changes must be made in equipment which is neither owned nor maintained by the computer network owner. This makes reconfiguration of the network difficult, since telephone company personnel must be coordinated with those of the network owner to effect a change.
Ideally, a network owner would like to be able to change the configuration of the network in response to observed error rates or the time of day. Such reconfiguration, for example, would be useful in smoothing peak loads on the computer network. Such reconfigurability would also be useful in transferring specific services to offices in earlier time zones when the office providing the service in another time zone closes at the end of the day. In prior art systems, such reconfiguration requires that key components of the computer network be rewired by telephone company personnel or that special purpose hardware be installed at significant cost.
Another example of a drawback in prior art computer networks is the inability of the user to monitor and manage the computer network. If a failure occurs in the computer network, it is difficult to ascertain whether the failure is the result of a malfunction in the equipment supplied by the telephone company, either on or off of the network owner's premises, or in the equipment owned and maintained by the network owner. Hence, when a malfunction is detected, telephone company technicians are often needed to ascertain the location of the failure within the system. The time delays in waiting for such determinations to be made are often unacceptable. It would be advantageous to provide a means for allowing the network user to set loopbacks and other diagnostic aids within the computer network without the time delays and added expense inherent when telephone company personnel must be involved. Similarly, the computer network user would like to be able to access data showing error rates on the various communication links in the computer network.
A typical multipoint network has a master computer and a plurality of slave computers. Each computer is connected to the telephone central office serving the geographic area in which the computer is located by subscriber loops which are, in general, identical to the communication lines used to connect a subscriber's telephone to the central office in question. Data is transferred between central offices with the aid of an apparatus, referred to hereinafter as an office control unit, which couples the subscriber loops in question to a time-domain multiplexed communication link used for transferring voice communications between central offices.
Prior art office control units do not provide the adaptive and control capabilities described above. In general, they allow the user to select one of a small number of data transmission speeds. To overcome the high error rates on the time-domain multiplexed communication links, office control units which utilize error correcting codes have been used. However, the error correcting codes utilized limit the maximum data transmission speeds in the computer network.
Broadly, it is an object of the present invention to provide an improved office control unit for use in computer networks utilizing the telephone system.
It is another object of the present invention to provide an office control unit which provides error correction without limiting the data rates on the time-domain multiplexed communication links.
It is yet another object of the present invention to provide an office control unit which monitors errors and provides information on the monitored error rates to the network user.
It is a still further object of the present invention to provide an office control unit which can perform loopback operations under the control of the network user.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.